1. Field of the Invention
The present invention relates to a projection display device for projecting an image on an imager (a beam modulating element) onto a projection plane, and more particularly to a projection display device for enlarging and projecting projection light in an oblique direction.
2. Description of the Related Art
There has been commercialized and widely spread a projection display device (hereinafter, called as “projector”) for enlarging and projecting an image on an imager such as a liquid crystal panel onto a projection plane such as a screen. For instance, this sort of projectors have an arrangement that three transmissive liquid crystal panels as imagers are used, red, green, and blue light beams are modulated by the respective panels, and the modulated light beams are synthesized by a dichroic prism for projection onto a screen plane by a projection optical system.
For this sort of projectors, there is proposed a method for enhancing light use efficiency, considering transmittance (reflectance) characteristics of a dichroic prism with respect to a polarization direction. Specifically, an optical component for converting a polarization direction is disposed between a liquid crystal panel and a dichroic prism. With use of the optical component, green light beams transmitted through the dichroic prism are converted into P-polarized beams for incidence onto the dichroic prism; and red and blue light beams reflected on a dichroic mirror are converted into S-polarized beams for incidence onto the dichroic prism. Thus, by controlling the polarization direction of the light beams of the respective colors, the amount of light transmitted through the dichroic prism is increased.
As another type of the projector, there is also proposed an oblique projection type projector constructed in such a manner that a wide-angle projection optical system is used, and the propagating direction of projection light is tilted with respect to an optical axis of the projection optical system to shorten the distance between a screen and a projector main body.
For instance, there is proposed an arrangement of the oblique projection type projector constructed in such a manner that a wide-angle lens having a large angle of view is used as a projection optical system, and an imager and a screen are shifted in directions opposing to each other with respect to an optical axis of the projection optical system. Thereby, oblique projection is realized, while securing a shortened throw distance, and eliminating a distortion.
In another arrangement of oblique projection, a refractive optical system and a reflection plane are used as a projection optical system, an image on an imager is formed as an intermediate image at a position between the refractive optical system and the reflection plane, and the intermediate image is enlarged and projected by the reflection plane. Thereby, shortening the throw distance is realized.
In the aforementioned arrangements, a trapezoidal distortion may occur in a projected image, resulting from oblique projection. The trapezoidal distortion, however, can be suppressed by shifting the imager with respect to the optical axis of the projection optical system.
The throw distance can be shortened while enhancing light use efficiency, by combining the aforementioned oblique projection arrangements with the aforementioned arrangement of controlling the polarization direction with respect to each of the colors. In the above combined arrangement, it is necessary to shift the imager with respect to the optical axis of the projection optical system to suppress a trapezoidal distortion.
In the case where the imager is shifted, the light amount of a specific color may be seriously lowered, as a projection beam is moved from one end of a projection area toward the other end of the projection area. As a result, color non-uniformity may occur in a displayed image. The lowering of light amount and generation of color non-uniformity are remarkable, as the shift amount of the imager with respect to the optical axis of the projection optical system is increased.
Assuming that green light beams are projected onto a screen as P-polarized beams, and red and blue light beams are projected onto the screen as S-polarized beams, as described above, if the screen is a polarization screen for preventing contrast lowering, merely the beams of a specific color are reflected on the screen, which may obstruct display of a proper color image.
The aforementioned drawback may be suppressed by arranging a quarter wavelength plate on the exit side of a projection lens, for example. In the above arrangement, green light beams, and red and blue light beams emitted from the projection lens are converted into circularly polarized beams having different rotation directions by the quarter wavelength plate. Guiding the light beams of the respective colors to the screen as circularly polarized beams enables to uniformly reflect the light beams of the respective colors on a polarization screen, even if the screen is the polarization screen. Thus, a proper color image can be displayed on the screen.
The above arrangement, however, fails to suppress lowering the light amount of a specific color, resulting from shifting the imager with respect to the optical axis of the projection optical system, and generation of color non-uniformity resulting from lowering the light amount of a specific color.